This invention relates to the optical filtering of an optical signal contaminated by noise, and in particular to the filtering of such a signal having a spectral bandwidth significantly smaller than the spectral bandwidth of a channel within which that signal is constrained to lie.
Such a situation is liable to occur for instance in a wavelength division multiplexed (WDM) system. In such a system the spectral width of an individual channel is much wider than the actual spectral width of the signal that is being transmitted on that channel. This is a consequence of there being a tolerance upon the specified emission wavelength of optical sources, typically semiconductor lasers, employed for transmission of the signal traffic. These specified tolerances have to make allowance for such factors as ageing and the effects of temperature. In the case of a typical 10 Gbit/s WDM system, the signal bandwidth is 0.08 nm, while the channel bandwidth is a few nm. Although the signal itself may have a bandwidth of only 0.08 nm, it will normally be accompanied by broader bandwidth noise. Such noise may emanate from spontaneous emission from a laser source and, in the case of a system including optical amplifiers, from spontaneous emission from the amplifiers. For multiplexing or demultiplexing purposes, a transmission filter will normally be required to have a bandwidth equal to the channel bandwidth, in this instance typically 2 nm wide. Any attempt to attempt to attenuate the noise power extending over the spectral range of the channel will attenuate the signal by an equivalent amount, and so will provide no improvement of signal to noise ratio.